Ultrasonic imaging is rapidly becoming the diagnostic modality of choice for characterizing internalized structures. In particular, miniaturized transducers mounted on probes and catheters for diagnosing and characterizing internalized structures in vivo that are accessible via endovascular or laproscopic means are know in the art, e.g., the probe tip transducers disclosed in U.S. Pat. No. 5,070,882 of Bui, et al.
In Ryan et al., "A High Frequency Intravascular Ultrasonic Imaging System for Investigation of Vessel Wall Properties," 1992 IEEE Ultrason. Symp. (1992), pp. 1101-1105, there is disclosed a prototype imaging system based on a 42 MHz, 0.7.times.0.7 mm lead zirconate titanate transducer built into a tip of a 30 cm long hypodermic stainless steel tube. This transducer has an absorptive epoxy backing and a quarter wave polyvinydlene fluoride (PVDF) matching layer. The signal emitted by the transducer is focused by a parabolic aluminum mirror. However, the system only achieves an axial resolution of 55 microns, which is insufficient to detect anatomical structures such as elastic laminae within arterial walls or atherosclerotic plaque which may require axial resolution on the order of 20 to 30 microns or less.
The imaging system of Griffith et al., U.S. Pat. No. 5,115,814, discloses a device for intravascular tissue characterization having a transducer capable of rotating within a catheter via a drive cable. The catheter is advanced within a vessel to be imaged using a previously positioned guide wire, the guide wire being withdrawn after the catheter is positioned. The imaging probe is thereafter inserted into the guide catheter and operated to obtain images of the vessel under investigation. The transducer is excited by circuitry so as to radiate relatively short duration acoustic bursts into the tissue surrounding the probe assembly while the transducer is rotating. The transducer receives the resulting ultrasonic echo signals reflected by the surrounding tissue. Unfortunately, the system of Griffith et al. is also limited in resolution because it is unfocused, operates at 15-30 MHZ, and uses a ceramic transducer. Roth et al., U.S. Pat. No. 5,207,672, discloses another ultrasonic imager that uses miniature transducers mounted within a catheter unit. The device disclosed in Roth et al. uses a pair of miniature transducers, one of which functions as a narrowband ultrasonic transmitter operating at about 7.5 MHz, and a second which functions as an ultrasonic receiver. A single transducer, or an array of transducers, may alternatively be used. A scanning motor is used to rotate the transducers so that image information received from a plurality of angular positions can be received, processed, stored, and displayed. A processor controller provides signals to the transmitting transducer, which generates an acoustic signal in response thereto. The receiving transducer receives reflected acoustic signals, which are converted into signals that are amplified, and digitized. However, the imager disclosed in Roth et al. is not suitable for the detection of elastic laminae within arterial walls or other anatomical features in that it is a narrowband device apparently not capable of operating at the higher frequencies necessary to image tissue characteristics requiring very high axial resolution. Thus, it appears that there are no broad band transducers available which are capable of providing the axial resolution necessary to image certain types of discrete in vivo features.
In manufacturing a broad band transducer using standard microfabrication techniques, the use of inorganic piezoelectric materials such as lead zirconate titanate (PZT) or zinc oxide (ZnO) are disfavored because they are brittle, difficult to deposit, and limited in the total strain that they can achieve. However, polyvinylidene fluoride (PVDF) is an organic piezoelectric material that overcomes some of these problems, and has previously been used in ultrasonic transducers (e.g., Mo et at., "Micromachining for Improvement of Integrated Ultrasonic Transducer Sensitivity," IEEE Trans. on Elec. Dev., Vol. 37, No. 1, Jan., 1990, pp. 134-140). Several advantages of PVDF over the inorganic compounds PZT and ZnO are its lower piezoelectric coefficient and lower thermal and chemical resistance. However, once being extruded and poled to be made piezoelectric, PVDF sheets must be adhered mechanically to the silicon substrate, which is not a standard microfabrication technique. Alternatively, the copolymer of PVDF with trifluorethylene (PVDF-TREE) can be spin-cast from solution directly onto substrates and then poled to be made piezoelectric without requiting extrusion. Suspended piezoelectric membranes using PDVF-TrFE films on silicon wafers have been described by Rashidian et al. in "Integrated Piezoelectric Polymers for Microsensing and Microactuation Applications," DSC-Vol. 32, Micromechanical Sensors, Actuators, and Systems, ASME 1991, ppo 171-179. However, no attempt at modifying such integrated devices for medical imaging applications requiring high resolution has been reported, presumably because of the difficulty in providing an acoustic impedance matched backing for wideband pulse echo imaging. Additionally, no attempts at focusing a wideband acoustic microscope which is integrated into a planar structure have been reported.
The use of a planar-structure focusing lens in a reflection-mode acoustic microscope was proposed in Yamada et al., "Planar-Structure Focusing Lens for Operation at 200 MHz and its Application to the Reflection-Mode Acoustic Microscope," 1986 IEEE Ultrasonic. Syrup. (1986), pp. 745-748. The disclosed configuration requires a thin film ZnO transducer at one end of a 10 mm diameter, 12 mm long fused quartz rod. The opposite end of the rod is etched into a planar lens using a gas plasma created by a microwave electron cyclotron resonance reactive ion etching technique. By this technique, a 200 MHz lens having focal length F=1.5 mm, aperture diameter 3.0 mm and aperture angle 2.THETA.=90.degree. was prepared. However, the large size of the focusing lens is not readily adaptable to in vivo diagnostic use.
A smaller and thinner lens structure can be made by exciting a thin-plate acoustic transducer only in regions corresponding to the transmissive zones of a Fresnel zone plate (FZP) pattern. A transducer using this technique to focus acoustic waves in water at frequencies near 10 MHz has been reported in Farnow et at., "Acoustic Fresnel Zone Plate Transducers," App. Phys. Letters, Vol. 25, No. 12, Dec. 15, 1994, pp. 681-682. A PZT transducer having one full-face electrode and a zone plate electrode on the other face thereof results in a transducer having an intensity distribution with a half-width of as little as 8.8 mil in the plane of focus. The primary focus is at a distance of 0.67 in. in water. Although this transducer does not require a large quartz focusing lens, the reported focusing dimensions do not lend themselves to intravascular medical imaging applications, and the operating frequency of the transducer is too low to provide the wide bandwidth ultrasound signal needed to provide the axial resolution necessary for certain types of in vivo tissue characterization. A further discussion of the focusing properties of acoustic transducers utilizing FZP electrode patterns has been published in Sleva et al., "Design and construction of a PVDF Fresnel Lens," 1990 IEEE Ultrason. Sympo (1990), pp. 821-826.
The high electrical input impedance associated with the small device dimensions of a transducer required for intravascular imaging suggests that it would be highly advantageous to provide buffer amplifiers and switching circuitry as close as possible to the transducer to achieve adequate signal-to-noise ratios. It would thus be advantageous from a manufacturing standpoint if a wideband transducer suitable for detection of cardiovascular defects and having dimensions appropriate for a catheter could be manufactured and pre-focused using standard microfabrication techniques that permit electronics associated with the transducer to be processed together with the transducer on the same substrate. The device in U.S. Pat. No. 5,041,849 to Quate et al. discloses a fresnel lens manufactured using standard microfabrication techniques. However, this device is designed for high-efficiency, narrow bandwidth applications such as acoustic ink printing.
Thus, the development of a wideband ultrasonic transducer having an integrated Fresnel lens is therefore needed to overcome the disadvantages of pulse echo imaging with presently known transducers.